If you haven’t been to Finland before, you need to be prepared for different kinds of weather. Right now, in mid-April, it’s colder than usual (pictures are from the morning of April 15).
Don’t worry, lakes won’t be frozen in June, and it is certainly warmer than now, but day temperatures could be anything between 10-25C (50-85F).
There are several sites to check the weather patterns for the next week. These two are among the more reliable: https://www.yr.no/place/Finland/Western_Finland/Jyv%C3%A4skyl%C3%A4/long.html
We will also let you know about the weather a week before the conference.